(1) Field of the Invention
This invention relates to the field of electromagnetic radiation absorbing or attenuating devices and more specifically relates to the field of microwave radiation absorbers.
(2) Description of the Art
Electromagnetic radiation absorbing materials (RAMs) are useful in a range of applications. For example, multipath reflections of TV signals can cause image ghosting. Materials, such as ferrite tiles, that can attenuate reflected radiation can improve signal quality.
At airports surveillance radar signals reflect from airport buildings resulting in stray secondary radar emissions. To reduce these secondary emissions some airport buildings (e.g. the World Cargo Centre at Heathrow Airport, UK) are covered in radar absorbing materials.
A further use of RAMs is in the automotive industry. Current and future car designs often now include a car based radar system for aiding parking manoeuvres and monitoring car separation distances whilst en route (i.e. collision warning systems). Generally only straight line returns from the car in front/behind are desired and so absorbent coatings are used to cut down on spurious reflections.
Two common radiation absorbing systems are Salisbury screens (see U.S. Pat. No. 2,599,944) and magnetic paint based arrangements.
A Salisbury screen is essentially a lossy material, such as carbon fibre, which is typically held at a quarter-wavelength separation from a metallic backing surface by a dielectric spacing material. This arrangement suffers from being bulky and difficult to apply to objects of complex geometry. For a typical airport application the radiation in question has a free-space wavelength of approximately 30 cm giving a thickness of up to 7.5 cm for the screen (the exact thickness being dependent upon the refractive indices of the constituent materials).
Magnetic paint arrangements generally comprise a metallic backing surface covered with a layer of magnetic paint. This arrangement is not as thick as a Salisbury screen but is heavy.